Marina Buljan

Free-form optics for Fresnel-lens-based photovoltaic concentrators

The Concentrated Photovoltaics (CPV) promise relies upon the use of high-efficiency triple-junction solar cells (with proven efficiencies of over 44%) and upon high-performance optics that allow for high concentration concurrent with relaxed manufacturing tolerances (all key elements for low-cost mass production). Additionally, uniform illumination is highly desirable for efficiency and reliability reasons. All of these features have to be achieved with inexpensive optics containing only a few (in general no more than 2) optical elements. In this paper we show that the degrees of freedom using free-forms allow the introduction of multiple functionalities required for CPV with just 2 optical elements, one of which is a Fresnel lens.

Recent trends in concentrated photovoltaics concentrators’ architecture

Abstract. The field of concentrated photovoltaics (CPV) has met some remarkable advances in recent years. The continuous increase in conversion efficiency of multijunction solar cells and new advancements in optics have led to new demands and opportunities for optical design in CPV. This paper is a mini-review on current requirements for CPV optical design, and it presents some of the main trends in recent years on CPV systems architecture.

Improving performances of Fresnel CPV systems: Fresnel-RXI Köhler concentrator.

The optical design presented here has been done in order to achieve superior optical performance in comparison with the state-of-the-art Fresnel CPV systems. The design consists of a Photovoltaic Concentrator (CPV) comprising a Fresnel lens (F) as a Primary Optical Element (POE) and a dielectric solid RXI as a Secondary Optical Element (SOE), both with free-form surfaces (i.e. neither rotational nor linearly symmetric). It is the first time the RXI-type geometry has been applied to a CPV secondary. This concentrator has ultra-high CAP value ready to accommodate more efficient cells eventually to be developed and used commercially in future.

Ultra-High Efficiency, High-Concentration PV System Based On Spectral Division Between GaInP/GaInAs/Ge And BPC Silicon Cells

A novel HCPV nonimaging concentrator concept with high concentration (>500×) is presented. It uses the combination of a commercial concentration GaInP/GaInAs/Ge 3J cell and a concentration Back‐Point‐Contact (BPC) concentration silicon cell for efficient spectral utilization, and external confinement techniques for recovering the 3J cells reflection. The primary optical element (POE) is a flat Fresnel lens and the secondary optical element (SOE) is a free‐form RXI‐type concentrator with a band‐pass filter embedded it, both POE and SOE performing Kohler integration to produce light homogenization. The band‐pass filter sends the IR photons in the 900–1200 nm band to the silicon cell. Computer simulations predict that four‐terminal terminal designs could achieve ∼46% added cell efficiencies using commercial 39% 3J and 26% Si cells. A first proof‐of concept receiver prototype has been manufactured using a simpler optical architecture (with a lower concentration, ∼100× and lower simulated added efficiency), a...

Advanced PV concentrators

It is essential to obtain high values of tolerance for PV concentrators because manufacturing process always implies some accuracy errors. In this way, three new free-form concentrators are presented here, combining high geometric concentration and high tolerance (high acceptance angle). This is achieved by using the SMS3D design method, which is the most advanced method to design free-form surfaces in non-imaging optics. Uniform illuminance on the cell is important as well, for proper behavior and durability of the system, so our three designs will have homogenizer elements. We have added a homogenizer rod to one of the designs while for the other two Ko¿hler integrator configurations have been chosen. Concentration, acceptance angle and uniformity values obtained are shown in results section.

Super-resolution optics for virtual reality

In present commercial Virtual Reality (VR) headsets the resolution perceived is still limited, since the VR pixel density (typically 10-15 pixels/deg) is well below what the human eye can resolve (60 pixels/deg). We present here novel advanced optical design approaches that dramatically increase the perceived resolution of the VR keeping the large FoV required in VR applications. This approach can be applied to a vast number of optical architectures, including some advanced configurations, as multichannel designs. All this is done at the optical design stage, and no eye tracker is needed in the headset.

High performance Fresnel-based photovoltaic concentrator

It is essential to obtain high values of tolerance for CPV concentrators because manufacturing process always implies some accuracy errors. This paper presents the Fresnel Köhler concentrator (FK), an advanced optical concentrator comprising a Fresnel lens as a primary element and a refractive secondary element, both presenting free-form surfaces. This optic produces both, the desired light concentration and high tolerance (i.e. high acceptance angle), as well as an excellent light homogenization by Köhler integration simultaneously. A comparison between the FK and other current conventional Fresnel-based CPV concentrators is also presented, being our concentrator superior to its competitors in terms of tolerances, irradiance homogeneity and manufacturability.

Design of compact optical systems using multichannel configurations

Compacting devices is an increasingly demanding requirement for many applications in both nonimaging and imaging optics. “Compacting” means here decreasing the volume of the space between the entry and the exit aperture without decreasing the optical performance. For nonimaging optical systems, compact optics is mainly important for reducing cost. Its small volume means less material is needed for mass-production and small size and light weight save cost in transportation. For imaging optical systems, in addition to the mentioned advantages, compact optics increases portability of devices as well, which contributes a lot to wearable display technologies such as Head Mounted Displays (HMD). After reviewing the different techniques to design compact systems, we analyze here the multichannel strategies. These type of designs split the incoming bundle of rays in different sub-bundles that are optically processed (independently) and then recombined in a single outgoing bundle. The optics volume decreases rapidly with the number of sub-bundles. These designs usually need to be combined with freeform optics in order to get optimum performance.

Optical design through optimization using freeform orthogonal polynomials for rectangular apertures

With the increasing interest in using freeform surfaces in optical systems due to the novel application opportunities and manufacturing techniques, new challenges are constantly emerging. Optical systems have traditionally been using circular apertures, but new types of freeform systems call for different aperture shapes. First non-circular aperture shape that one can be interested in due to tessellation or various folds systems is the rectangular one. This paper covers the comparative analysis of a simple local optimization of one design example using different orthogonalized representations of our freeform surface for the rectangular aperture. A very simple single surface off-axis mirror is chosen as a starting system. The surface is fitted to the desired polynomial representation, and the whole system is then optimized with the only constraint being the effective focal length. The process is repeated for different surface representations, amongst which there are some defined inside a circle, like Forbes freeform polynomials, and others that can be defined inside a rectangle like a new calculated Legendre type polynomials orthogonal in the gradient. It can be observed that with this new calculated polynomial type there is a faster convergence to a deeper minimum compared to “defined inside a circle” polynomials. The average MTF values across 17 field points also show clear benefits in using the polynomials that adapted more accurately to the aperture used in the system.

Quasi-aplanatic free-form V-groove collimators for LED colour mixing

Two quasi-aplanatic free-form solid V-groove collimators are presented in this work. Both optical designs are originally designed using the Simultaneous Multiple Surface method in three dimensions (SMS 3D). The second optically active surface in both free-form V-groove devices is designed a posteriori as a grooved surface. First two mirror (XX) design is designed in order to clearly show the design procedure and working principle of these devices. Second, RXI free-form design is comparable with existing RXI collimators; it is a compact and highly efficient design made of polycarbonate (PC) performing very good colour mixing of the RGGB LED sources placed off-axis. There have been presented rotationally symmetric non-aplanatic high efficiency collimators with colour mixing property to be improved and rotationally symmetric aplanatic devices with good colour mixing property and efficiency to be improved. The aim of this work was to design a free-form device in order to improve colour mixing property of the rotationally symmetric non-aplanatic RXI devices and the efficiency of the aplanatic ones.